Many fibers are prepared from a solution of the polymer in a solvent (called the “polymer dope”) by extruding or spinning the polymer dope through a die or spinneret to prepare or spin a dope filament. The solvent is subsequently removed to provide the fiber or yarn. In the preparation of certain fibers, the solvent utilized is a solvent acid, such as polyphosphoric acid (PPA). Unlike many typical solvents, PPA removal is generally more difficult, in part due to its polymeric nature. Incorporation of heteroatoms into the polymer may also act to inhibit removal of polyphosphoric acid from the fiber or yarn. Existing processes for removal of polymeric PPA solvent from a polymeric material typically require long washing times or elevated leaching temperatures if a substantial amount of PPA is to be removed.
For example, Sen et al., U.S. Pat. No. 5,393,478, discloses a process for leaching polyphosphoric acid from the polybenzazole dope filament by contacting with a leaching fluid at a temperature of at least about 60° C.
Sen et al., U.S. Pat. No. 5,525,638, discloses a process for washing polyphosphoric acid from the polybenzazole dope filament by using multiple washes, typically at about room temperature, slowly reducing phosphorous concentration from the spun fiber, allegedly to improve the physical properties of the resultant polymeric fiber.
Further improvements in the physical properties of and/or removal of phosphorous from fibers spun from polyphosphoric acid are needed. These and other objects of the invention will become more apparent from the present specification and claims.